The invention generally relates to an arrangement for measuring temperatures of workpieces to be treated in a corona or glow discharge field.
More particularly, the invention relates to an arrangement for measuring temperatures of workpieces to be case-hardened in the presence of a hardening agent, to be carburized, decarburized, metalized or the like.
Still more particularly, the invention relates to an arrangement for measuring temperatures of workpieces to be nitrided in the presence of a nitrogeneous atmosphere, commonly ammonia gas.
Even more particularly, the invention relates to an arrangement for preventing short-circuiting between a workpiece to be nitrided and a container in which the workpiece is accommodated.
The nitrogen case-hardening process which is termed "nitriding" consists in subjecting machined and preferably heat-treated parts to the action of a nitrogeneous medium, commonly partially dissociated ammonia gas, under certain conditions whereby surface hardness is imparted to the material without necessitating any further treatment. The nitriding process imparts to the metal to be case-hardened vastly improved properties such as increased wear resistance, retention of hardness at elevated temperatures, and increased resistance to certain types of corrosion.
It has been proposed to nitride a workpiece by electrically connecting a container in which a workpiece is accommodated to a positive terminal of an electrical current source, and by electrically connecting the workpiece itself to a negative terminal of the electrical current source. The source is of sufficient strength to establish an electrical potential or voltage gradient between the container and the workpiece so as to heat the workpiece and concomitantly produce about the exterior surface of the workpiece, i.e. in the circumambient region of the workpiece, a corona or glow discharge field of ionized particles including nitrogen ions which interact with the surface of the workpiece to thereby penetrate into and harden the exterior surface area of the latter. A corona or glow discharge is a discharge brought about as a result of ionization of a gas medium surrounding a conductor and occurs when the potential gradient exceeds a certain value.
It has further been proposed to measure the temperature to which the workpiece is heated by inserting a thermal sensor element, such as a thermocouple, in a bore either already available or deliberately machined in the workpiece. The thermocouple wires conduct an electrical signal generated from the temperature detected at the heated workpiece in direction along a path from the workpiece to the exterior of the container and thereupon to an indicating and/or measuring instrument.
However, such wires, even though surrounded by a protective tubular member, must pass through the corona discharge field. This results in the decided disadvantage that metal particles spattered from the workpiece because of the bombardement of the ionized particles tend to deposit themselves along the path defined by the wire and the tubular member. In time, a continuous electrically conductive path comprised of such deposited particles-would form intermediate the container and the workpiece. This results in a short-circuit being formed between the container and the workpiece. It will be appreciated that the formation of such a continuous electrically-conductive path also results in the discontinuance of the electrical potential gradient between the container and the workpiece, thus terminating the case-hardening or nitriding operation.